1. Field of the Invention
This invention relates to a local oscillating device, and more particularly to a local oscillating device for, e.g., INMARSAT STD-C (International Maritime Satellite Organization Standard-C) system.
2. Prior art
International trans-ocean communications, i.e., communications between a mobile earth station and a coast earth station or communication between mobile earth stations are dependent upon short wave. However, these international transocean communications suffer from communication time zone and/or the coverage restrictions which render them somewhat inadeguate in terms of communication quality and the circuit capacity. In view of this, a so-called INMARSAT (Inter national Maritime Satellite Organization) was inaugurated in 1979. As a result, the use of a Maritime Satellite Communication System (hereinafter referred to as INMARSAT system) has been used in the Atlantic Ocean since 1982.
This INMARSAT system enables communication between a mobile earth station and a coast earth station or between mobile earth stations relaying a geostationary satellite or a mobile satellite, and is composed of a Network Coordination Station (NCS) for controlling the satellite, the circuit or traffic, a Coast Earth Station (CES) on the earth side, and a Mobile Earth Station (MES) on the ship side, and so on.
Further, the above-mentioned mobile earth station is classified into a standard A station, a standard B station, a standard C station and a standard D station in accordance with the service provided.
More specifically, the standard A station uses a parabolic antenna having a diameter of about 1.2 m, and has the capability of carrying out low speed data communication such as a telephone or a telex, etc., audio band data communication, and high speed data (mobile earth station-coast earth station) of 56K bps. The standard B station is directed to small ship communications. This station aims at utilizing a small economical shipboard antenna (short backfire antenna having a diameter of about 0.4 m) to enable low speed data communication such as a telephone or a telex, etc. The standard C station is also directed to small ship communications wherein a short backfire antenna or a dipole antenna, for example, is used. This station is principally directed toward distress safety communications, and has the capability of carrying out low speed data commnication such as distress safety communication or telex, etc. Finally, the standard D station includes a parabolic antenna having diameter of 2.5 to 3 m which is larger than the above-mentioned standard A station, and which has the capability of carrying out high speed data communicatins, such as multiple telephone communications etc., with oil rigs and the like.
Meanwhile in the INMARSAT system, the frequencies used are such that 1.6/1.5 GHz band is used between the satellite and the mobile earth station, and 6/4 GHz band is used between the satellite and the coast earth station. Further, the so-called Demand assigned system is employed to assign the circuit to a desired station when traffic (message) transmission occurs. For telephones, the Frequency Modulation/Time Division Multiple Access (FM/TDMA) system is used. For low speed data such as telex, etc., Time Division Multiples (TDM) system coast earth station-mobile earth station) and TDMA system mobile earth station-coast earth station) based on Binary Phase Shift Keying (BPSK), are used.
The system of the standard C station of the INMARSAT (hereinafter simply referred to as INMARSAT STD-C system) will now be briefly described.
The INMARSAT STD-C system is composed, as shown in FIG. 2, of Circuit Terminating Equipment (DCE) 50 which interfaces with a satellite, and a Data Terminal Equipment (DTE) 80 which has a user interface. For example, when a message (data) is transmitted from a mobile earth station to a land station, data from the terminal, e.g., a word processor is subjected to a predetermined formatting at the DTE 80. Thereafter, error correction encoding and/or BPSK modulation, etc. is applied to the formatted data at the DCE 50. The data thus processed, is transmitted to the satellite using a frequency band of 1.6 GHz. Thereafter, the data is converted to a frequency band of 4 GHz. and is then amplified at the satellite. The data thus obtained is transmitted to the coast earth station. On the other hand, when sending a message (data) from a coast earth station to a mobile earth station, data is transmitted in a frequency band of 6 GHz from the coast earth station to the satellite. At the satellite, this data is converted to a frequency band of 1.5 GHz and is then amplified. The data thus obtained is transmitted to the DCE 50 of the mobile earth station. At the DCE 50, BPSK modulation and/or error correction, etc. are applied to the data, resulting in reproduced data. This reproduced data is sent to the terminal through the DTE 80.
As shown in FIG. 2, the DCE 50 is composed of an antenna (ANT) 51, a transmitting system circuit 60, a receiving system circuit 70, a changeover switch 52 for switching the antenna 51 between transmitting and receiving modes, a local oscillating circuit (SYNTH) 53 for delivering a carrier and a clock, etc. to the transmitting system circuit 60 and the receiving system circuit 70, and a controller 54 for establishing access control and/or handling messages.
More particularly, the transmitting system circuit 60 comprises a scrambler 61 for implementing pseudo random coding which power diffuses data from the controller 54, a convolutional encoder 62 for implementing a convolutional coding for carrying out error correction, an interleaving circuit 63 for implementing interleaving which converts burst error to random error, a BPSK modulation circuit 64 for carrying out binary phase shift keying, a multiplier 65 for converting a BPSK-modulated signal to a transmitting frequency signal, and a high power amplifier (HPA) 66 for amplifying the transmitting signal.
On the other hand, the receiving system circuit 70 comprises a low noise amplifier (LNA) 71 for amplifying a received signal from the antenna 51, a multiplier 72 for converting the received signal to an intermediate frequency signal, an IF circuit 73 for carrying out amplfication of the intermediate frequency signal, or the like, a BPSK demodulation circuit 74 for demodulating a BPSK modulated signal, a frame synchronous circuit 75 for pulling in frame of TDM, a deinterleaving circuit 76 for transforming interleaved data to data in its original form, a Viterbi decoder 77 for carrying out error correction of convolutionally coded data, and a descrambler 78 for transforming pseudo random coded data to data in its original form.
Meanwhile, demodulation (synchronous detection) in the BPSK modulation system is performed as follows. Namely, a phase comparison between a carrier, serving as reference, and a received signal is carried out. If the two are in phase, "1" is allocated, whereas if they are in out of phase, "0" is allocated. The demodulation is thus performed by making judgment of "0" and "1" using a symbol clock which is reproduced from the received signal.
In the INMARSAT STD-C system, data is received in burst form each frame. The carrier wave used for demodulation is provided by reproducing so called Carrier Recovery (CR) which is added, for example, to the leading portion of the frame, and transmitted therewith. Further, the above-mentioned symbol clock is provided by reproducing so called Bit Timing Recovery (BTR) similarly added (e.g., to the leading portion of the frame) and transmitted. Thus, demodulation is performed using reproduce the carrier and symbol clock, which have been reproduced, serving as reference.
In more actual terms, the BPSK demodulation circuit 74 comprises a multiplier for converting an intermediate frequency signal (hereinafter simply referred to as an IF signal) from IF circuit 73 to a BPSK modulated signal, and a BPSK demodulator.
The multiplier, i.e., the mixer within this BPSK demodulation circuit 74 serves to convert an IF signal to a BPSK modulated signal by using a carrier delivered from the local oscillating circuit 53, i.e., so-called carrier for down conversion.
Further, in the case of carrying out the BPSK demodulation by digital processing, the BPSK demodulator within the above-mentioned BPSK demodulation circuit 74 comprises an analog-to-digital converter (hereinafter referred to as an A/D converter) for converting a BPSK modulated signal to a digital signal, and a so-called digital signal processor (hereinafter referred to as a DSP) for digitally carrying out demodulation processing, etc. Viz., this A/D converter converts a BPSK modulated signal to a digital signal byusing a sampling clock from, for example, a quartz-crystal oscillator provided within the local oscillating circuit 53. Further, the DSP reproduces the above-described carrier, which serves as a reference for demodulation, from the converted BPSK modulated signal, and carries out (by way of example) synchronous detection by using the reproduced carrier to further reproduce the above-described symbol clock which is necessary to carry out judgment the "0" and "1" determination, and digitally execute the BPSK demodulation.
Meanwhile, in order to simplify demodulation digital processing in the DSP, it is necessary to bring the sampling clock of the A/D converter into synchronism with the above-mentioned reproduced symbol clock. To this end, a finely adjusted quartz-crystal oscillator is used as the quartz-crystal oscillator the above-mentioned oscillating circuit 53. However, even when the quartz-crystal oscillator is used, frequency accuracy of less than 1 ppm cannot be realized.
Further, reproduction of carrier or symbol clock in the DSP requires the following. Namely, Carrier Recovery and Bit Timing Recovery from the leading portion of a signal received in a burst format are used to carry out rapid reproduction so that an erroneous pulling into an erroneous phase or so called "slip", does not take place. Various devices which complicate the circuit configuration are required to achieve this. For example, circuitry is required to meet the conflicting requirements wherein a so-called Q parameter is required to be set to a high value in order to decrease the possibility that slip occurs, while being attenuated in order to avoid reproduction time being prolonged.